A hydrogen generator that generates a reformed gas mainly containing hydrogen by steam-reforming of a hydrocarbon-based feed gas such as town gas and LP gas is used for, for example, producing hydrogen that is used as a feed gas in a fuel cell. Since the steam reforming reaction (hereinafter simply referred to as “reforming reaction”) in the hydrogen generator is an endothermic reaction, it is necessary to keep a reformer at a temperature of about 550° C. to 800° C. in order to maintain the reforming reaction. For this reason, in the hydrogen generator, a heating source such as a burner is installed, and a high-temperature combustion gas obtained from the heating source, a radiant body that emits the radiant heat of the combustion gas, or the like, is utilized to heat the reformer.
Meanwhile, the reformed gas obtained with the reformer of the hydrogen generator mainly contains hydrogen, as mentioned above, but also contains CO as a byproduct produced in the reforming reaction. If such a reformed gas containing CO is directly supplied to the fuel cell, the CO reduces the reactivity of catalysts in the fuel cell. For this reason, in order to remove CO, a shifter that converts CO contained in the reformed gas into CO2 through a shift reaction is disposed downstream of the reformer in the hydrogen generator.
As a conventional hydrogen generator, there is one in which a heat insulator is arranged along the outer circumference of a wall surface of a reformer, and a shifter is arranged so as to surround the reformer with the heat insulator interposed therebetween so as to inhibit heat from dissipating to outside from the reformer that is heated to a high temperature as described above (for example, cf. Japanese Patent. No. 3108269 (pp. 2-4, FIG. 3)). In addition, there is a hydrogen generator having a configuration in which a plurality of cylindrical units are concentrically arranged upright, a reformer is formed by filling a reforming catalyst into one of the cylinder-shaped clearance spaces formed by wall surfaces of the cylindrical units, and a shifter is formed by filling a CO shifting catalyst into a clearance space that is located on the outer circumference of the reformer; in such a configuration, the circumference of the reformer is covered by the shifter which is kept at a lower temperature (about 180° C. to 400° C.) than that of the reformer, and consequently, heat transfers from the reformer to the shifter. As a result, it is possible to decrease heat dissipation to outside (for example, cf. Japanese Laid-Open Patent Application Publication No. 2002-187705 (pp. 5-10, FIG. 1)).
However, in the hydrogen generators with the above-described configurations, heat transfers mainly from the reformer to the shifter since the reformer is higher in temperature than the shifter, and meanwhile, the heat dissipating from the shifter substantially does not transfer to the reformer. Thus, the heat that has dissipated from the shifter cannot be utilized effectively by returning it to the reformer, and as a result, sufficiently high thermal efficiency cannot be attained.